SEM Photography and Science Photos | Articles by PSmicrographs

House dust mites.

 

A brief study by Scanning Electron Microscopy into these tiny creatures.



 
 

So wrote Robert Hooke in 1665 in his widely acclaimed work `Micrographia' dedicated to King Charles II amidst the last great plague of London and a year before the great fire which destroyed 13,000 buildings but surprisingly claimed the lives of only four people. Certainly fraught times for a Londoner to live through, whilst concerning himself with dust mites.


The house dust mite is neither a reptile nor insect but until Carolus Linnaeus brought some assemblage of order into classification in 1757 with his Systema Naturae, a scheme of binomial nomenclature, little serious attention was paid to such detail. Employing his basic microscope with reletively poor lenses, Hooke described the mites in considerable detail with remarkable accuracy carefully stressing the ".. eight well shaped and proportioned legs ..." if comparing them still further with crabs and lobsters. Had he lived to see the 300th anniversary of his work, he would have witnessed the marketing of the first commercially available scanning electron microscope, by Cambridge Instruments, which surely would have astounded him.


Modern classification places mites with the spiders and though their classification appears to be in a state of flux, house dust mites may be classified as follows:


Phylum - Arthropoda 

Class - Arachnida

Order - Acarina

Sub-Order - Sarcoptiformes

Division - Astigmata

Family - Epidermoptidae

Genus - Dermatophagoides

Species - D. farinae, D. pteronyssinus


It is alleged that house dust mites inhabit our homes by the millions; our beds, furniture and carpets are over-run with them. Manufacturers of vacuum cleaners and producers of mite-proof, `anti-allergenic' fabrics for bedding materials market their products by trading excessively on this general perception. Adult dust mites are very small, approximately 300um in length, so head to tail three or four approximately span a millimetre and are almost invisible to the unaided eye under normal conditions. Males tend to be smaller and more flattened than the female which takes on a rotund appearance especially when fully mature and carrying eggs.

 


The tough leathery skin is mostly translucent allowing the internal organs and haemolymph to impart an overall creamy white appearance to the body with isolated patches of pale yellow. Sclerotised areas, such as the legs and head of fully mature adults, are more heavily pigmented with a red/brown colouration that relieves the otherwise bland exterior. Microscopically the most striking and aesthetically pleasing feature of the skin is the sculpturing that resembles whorls of a finger-print.

 

 


The house dust mite is devoid of a true head, this has evolved into a complex mouthparts structure known as the gnathosoma with which it feeds on hair debris, flakes of skin and general detritus.

 

Eyes are also absent but on each side of the body immediately above the first pair of legs there exists a bristle, the supra-coxal setum, associated with an elongated pore located in a recess . The bristle's architecture varies considerably between the genera of mites (being the least spectacular in Dermatophagoides spp.) and for the acarologist is useful for identification purposes but undoubtedly is a highly developed sensory organ essential for the mites survival. Though there are no obvious light receptors house dust mites are extremely photophobic and become very animated to seek dark recesses when brought into the light.

 

The life-cycle of the house dust mite is a three stage process; egg, larva (nymph) and adult. As there is no distinct pupal stage, metamorphosis is said to be incomplete. The larval stage however is protracted and as the nymph develops and grows, it sheds its skin three times before becoming a fully formed, sexually mature adult. From a visual perspective nymphs closely resemble a smaller version of the adult form with two notable exceptions; they are mostly devoid of colour and more obviously have only three pairs of legs. Though visible as `buds' during the third nymphal instar, the hind (fourth) pair of legs is not present until the adult mite emerges from the final ecdysis. The entire cycle from egg to adult spans 2 to 3 weeks, with the adult living for about a month, but is very variable depending upon the ambient temperature and relative humidity of the environment.

 

 

House dust mites can tolerate a wide temperature range from around 0 degrees C (in a moribund condition) to a maximum temperature of about 30 degrees C with 20 - 25 degrees C being the optimum for breeding. Humidity of the environment is more important and there is less tolerance to variation where the optimum relative humidity is about 75% with a maximum deviation of +/- 10%. Whilst most centrally heated homes probably maintain a constant ideal temperature the humidity generally will be lower than 65% perhaps not making these types of buildings as attractive to the mite as one is led to believe. 

 

Sexually mature adults mate by the male initially climbing onto the  back of the female with both individuals facing the same direction but the male quickly reorientates itself through 180 degrees to take up a retroconjugate mating position. In nearly all cases of matings observed by the author this activity resulted in the female becoming very quiet and totally immobile as though influenced by a male initiated pheromone.

 


The male maintains position on the back of the female by two large paranal suckers on the underside of the abdomen.

 

  

 

The copulatory organ of the male is located approximately half way down the body on the ventral surface and protected by two paragynal lips (upper image, left) through which the penis protrudes by haemolymphatic pressure during copulation enabling penetration of the genital/anal pore (upper image, right) of the female.Once insemination has occurred, the male abandons the female which, from direct observations, appears to take tens of minutes to recover full mobility.

 

 

The female subsequently lays white, relatively large, eggs singly at random by extrusion from the ovipositor, located centrally in the ventral surface, which hatch after a short incubation period of two or three days depending upon environmental conditions.

 

 

House dust mites have been identified as a source of allergens for those individuals sensitive or allergic to house dust components. It has now been established that rather than the mite itself as the causative agent of the reaction, it is the faecal matter the mite excretes and more specifically a residual enzyme it contains that is responsible for most allergies. Faecal pellets are approximately 10um to 20um in diameter, the same size or smaller than many pollen grains, dry and susceptible to becoming statically charged. If the pellets are accidentally disturbed into the air they can remain airborne for long periods of times and in this respect have similar properties to pollen, all too familiar allergens of hay fever.


Survival Against All Odds




It was the summer of 2009 and we were in the garden and as most most people know, cabbage white butterfly eggs on the leaves of your Brassicas is a gardener’s nightmare.  When hatched into caterpillars, these veracious eaters will devour your cabbages before you can say meat and two veg (less cabbage).  Having these tiny eggs in our garden and being in possession of a scanning electron microscope (in the lounge would you believe), we decided that some of these eggs could be sacrificed in the name of science.   

This exercise is rather time consuming as the eggs have to be carefully handled and prepared so they can stand the rigors of the vacuum chamber of the electron microscope.  After carefully removing the eggs from the brassica leaf they were placed in Osmium Tetroxide vapour for ½ hour.  This is a highly toxic substance which makes tissue go black and fixes proteins.  The next process is to coat the eggs in gold to make them conductive so they don’t “charge” in the electron microscope.  This process is called “sputter coating” which has to be done under vacuum in a sputter coater machine. Charging will ruin an image and damage the sample so sputter coating of tissue is a must for electron microscopy.

 

The gold coated eggs were then placed in the scanning electron microscope and put under a high vacuum (for those interested  7.5x10-5 Torr).  The eggs were in this vacuum for 5 hours whilst we were taking various images of them.  Apart from the vacuum, they had to withstand being irradiated with a 10kv electron beam where X-Rays are generated when the electrons hit the target in the chamber.

After 5 hours, the electron microscope was brought back to atmospheric pressure and the eggs were removed and left on the side.  Next morning we noticed something moving, the eggs were starting to hatch.  8 of the caterpillars survived.  They were put onto nasturtians as a food plant but 5 subsequently died after a few days.  The remaining 3 carried on to pupate and in the following spring (March) all 3 emerged from the chrysalis, apparently fit and well as you can see by the 3 time lapse videos on this site.



About Scanning Electron Microscopes (SEM)

 

The very first scanning electron micrograph was obtained in 1935 by Max Knoll and the principles of the sem were further pioneered by Manfred von Ardenne in 1937 but it wasn’t until 1965 that the first sem was produced by the Cambridge Instrument Company, known as the “Stereoscan”.

The scanning electron microscope (sem) works by scanning a focused beam of electrons across the surface of the specimen which generates both secondary and back scattered electrons. These electrons are then collected by suitable detectors to form an image of the sample which is displayed onto a suitable screen.